Re: [PATCH 0/6] CLONE_FD: Task exit notification via file descriptor

From: David Drysdale
Date: Fri Mar 13 2015 - 12:05:58 EST

On Fri, Mar 13, 2015 at 1:40 AM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
> This patch series introduces a new clone flag, CLONE_FD, which lets the caller
> handle child process exit notification via a file descriptor rather than
> SIGCHLD. CLONE_FD makes it possible for libraries to safely launch and manage
> child processes on behalf of their caller, *without* taking over process-wide
> SIGCHLD handling (either via signal handler or signalfd).

Hi Josh,

>From the overall description (i.e. I haven't looked at the code yet)
this looks very interesting. However, it seems to cover a lot of the
same ground as the process descriptor feature that was added to FreeBSD
in 9.x/10.x:

I think it would ideally be nice for a userspace library developer to be
able to do subprocess management (without SIGCHLD) in a similar way
across both platforms, without lots of complicated autoconf shenanigans.

So could we look at the overlap and seeing if we can come up with
something that covers your requirements and also allows for something
that looks like FreeBSD's process descriptors?

(I've actually got some rough patches to add process descriptor
functionality on Linux, so I can look at how the two approaches compare
and contrast.)

> Note that signalfd for SIGCHLD does not suffice here, because that still
> receives notification for all child processes, and interferes with process-wide
> signal handling.
> The CLONE_FD file descriptor uniquely identifies a process on the system in a
> race-free way, by holding a reference to the task_struct. In the future, we
> may introduce APIs that support using process file descriptors instead of PIDs.

FreeBSD has pdkill(2) and (theoretically) pdwait4(2) along these lines.
I suspect we need either need pdkill(2) or a way to retrieve a PID from
a process file descriptor, so that there's a way to send signals to the

> Introducing CLONE_FD required two additional bits of yak shaving: Since clone
> has no more usable flags (with the three currently unused flags unusable
> because old kernels ignore them without EINVAL), also introduce a new clone4
> system call with more flag bits and an extensible argument structure. And
> since the magic pt_regs-based syscall argument processing for clone's tls
> argument would otherwise prevent introducing a sane clone4 system call, fix
> that too.
> I tested the CLONE_SETTLS changes with a thread-local storage test program (two
> threads independently reading and writing a __thread variable), on both 32-bit
> and 64-bit, and I observed no issues there.

Worth preserving in tools/testing/selftests/ ?

> I tested clone4 and the new CLONE_FD call with several additional test
> programs, launching either a process or thread (in the former case using
> syscall(), in the latter case by calling clone4 via assembly and returning to
> C), sleeping in parent and child to test the case of either exiting first, and
> then printing the received clone4_info structure. Thiago also tested clone4
> with CLONE_FD with a modified version of libqt's process handling, which
> includes a test suite.
> I've also included the manpages patch at the end of this series. (Note that
> the manpage documents the behavior of the future glibc wrapper as well as the
> raw syscall.) Here's a formatted plain-text version of the manpage for
> reference:

FYI, I've added some comparisons with the FreeBSD equivalents below.

> CLONE4(2) Linux Programmer's Manual CLONE4(2)
> clone4 - create a child process
> /* Prototype for the glibc wrapper function */
> #define _GNU_SOURCE
> #include <sched.h>
> int clone4(uint64_t flags,
> size_t args_size,
> struct clone4_args *args,
> int (*fn)(void *), void *arg);
> /* Prototype for the raw system call */
> int clone4(unsigned flags_high, unsigned flags_low,
> unsigned long args_size,
> struct clone4_args *args);
> struct clone4_args {
> pid_t *ptid;
> pid_t *ctid;
> unsigned long stack_start;
> unsigned long stack_size;
> unsigned long tls;
> };
> clone4() creates a new process, similar to clone(2) and fork(2).
> clone4() supports additional flags that clone(2) does not, and accepts
> arguments via an extensible structure.
> args points to a clone4_args structure, and args_size must contain the
> size of that structure, as understood by the caller. If the caller
> passes a shorter structure than the kernel expects, the remaining
> fields will default to 0. If the caller passes a larger structure than
> the kernel expects (such as one from a newer kernel), clone4() will
> return EINVAL. The clone4_args structure may gain additional fields at
> the end in the future, and callers must only pass a size that encomâ
> passes the number of fields they understand. If the caller passes 0
> for args_size, args is ignored and may be NULL.
> In the clone4_args structure, ptid, ctid, stack_start, stack_size, and
> tls have the same semantics as they do with clone(2) and clone2(2).
> In the glibc wrapper, fn and arg have the same semantics as they do
> with clone(2). As with clone(2), the underlying system call works more
> like fork(2), returning 0 in the child process; the glibc wrapper simâ
> plifies thread execution by calling fn(arg) and exiting the child when
> that function exits.
> The 64-bit flags argument (split into the 32-bit flags_high and
> flags_low arguments in the kernel interface) accepts all the same flags
> as clone(2), with the exception of the obsolete CLONE_PID,
> CLONE_DETACHED, and CLONE_STOPPED. In addition, flags accepts the folâ
> lowing flags:
> Instead of returning a process ID, clone4() with the CLONE_FD
> flag returns a file descriptor associated with the new process.
> When the new process exits, the kernel will not send a signal to
> the parent process, and will not keep the new process around as
> a "zombie" process until a call to waitpid(2) or similar.
> Instead, the file descriptor will become available for reading,
> and the new process will be immediately reaped.

Just to confirm: presumably a waitpid(-1,...) call that's already in
progress won't return when one of these child processes exits?

> Unlike using signalfd(2) for the SIGCHLD signal, the file
> descriptor returned by clone4() with the CLONE_FD flag works
> even with SIGCHLD unblocked in one or more threads of the parent
> process, and allows the process to have different handlers for
> different child processes, such as those created by a library,
> without introducing race conditions around process-wide signal
> handling.
> clone4() will never return a file descriptor in the range 0-2 to
> the caller, to avoid ambiguity with the return of 0 in the child
> process. Only the calling process will have the new file
> descriptor open; the child process will not.

FreeBSD's pdfork(2) returns a PID but also takes an int *fdp argument to
return the file descriptor separately, which avoids the need for special
case processing for low FD values (and means that POSIX's "lowest file
descriptor not currently open" behaviour can be preserved if desired).

> Since the kernel does not send a termination signal when a child
> process created with CLONE_FD exits, the low byte of flags does
> not contain a signal number. Instead, the low byte of flags can
> contain the following additional flags for use with CLONE_FD:
> Set the O_CLOEXEC flag on the new open file descriptor.
> See the description of the O_CLOEXEC flag in open(2) for
> reasons why this may be useful.
> Set the O_NONBLOCK flag on the new open file descriptor.
> Using this flag saves extra calls to fcntl(2) to achieve
> the same result.
> clone4() with the CLONE_FD flag returns a file descriptor that
> supports the following operations:
> read(2) (and similar)
> When the new process exits, reading from the file
> descriptor produces a single clonefd_info structure:
> struct clonefd_info {
> uint32_t code; /* Signal code */
> uint32_t status; /* Exit status or signal */
> uint64_t utime; /* User CPU time */
> uint64_t stime; /* System CPU time */
> };

Presumably there is no way to get full rusage information for the exited

[FreeBSD theoretically has pdwait4(2) to do wait4-like operations on a
process descriptor, including rusage retrieval. However, I don't think
they actually implemented it:]

> If the new process has not yet exited, read(2) either
> blocks until it does, or fails with the error EAGAIN if
> the file descriptor has been made nonblocking.
> Future kernels may extend clonefd_info by appending addiâ
> tional fields to the end. Callers should read as many
> bytes as they understand; unread data will be discarded,
> and subsequent reads after the first will return 0 to
> indicate end-of-file. Callers requesting more bytes than
> the kernel provides (such as callers expecting a newer
> clonefd_info structure) will receive a shorter structure
> from older kernels.

FreeBSD also implements fstat(2) for its process descriptors, although
only a few of the fields get filled in.

> poll(2), select(2), epoll(7) (and similar)
> The file descriptor is readable (the select(2) readfds
> argument; the poll(2) POLLIN flag) if the new process has
> exited.

FreeBSD uses POLLHUP here.

> close(2)
> When the file descriptor is no longer required it should
> be closed. If no process has a file descriptor open for
> the new process, no process will receive any notification
> when the new process exits. The new process will still
> be immediately reaped.

FreeBSD has two different behaviours for close(2), depending on a flag
value (PD_DAEMON). With the flag set it's roughly like this, but
without PD_DAEMON a close(2) operation on the (last open) file
descriptor terminates the child process.

This can be quite useful, particularly for the use case where some
userspace library has an FD-controlled subprocess -- if the application
using the library terminates, the process descriptor is closed and so
the subprocess is automatically terminated.

> C library/kernel ABI differences
> As with clone(2), the raw clone4() system call corresponds more closely
> to fork(2) in that execution in the child continues from the point of
> the call.
> Unlike clone(2), the raw system call interface for clone4() accepts
> arguments in the same order on all architectures.
> The raw system call accepts flags as two 32-bit arguments, flags_high
> and flags_low, to simplify portability across 32-bit and 64-bit archiâ
> tectures and calling conventions. The glibc wrapper accepts flags as a
> single 64-bit argument for convenience.
> For the glibc wrapper, on success, clone4() returns the file descriptor
> (with CLONE_FD) or new process ID (without CLONE_FD), and the child
> process begins running at the specified function.
> For the raw syscall, on success, clone4() returns the file descriptor
> or new process ID to the calling process, and returns 0 in the new
> child process.
> On failure, clone4() returns -1 and sets errno accordingly.
> clone4() can return any error from clone(2), as well as the following
> additional errors:
> EINVAL flags contained an unknown flag.
> EINVAL flags included CLONE_FD, but the kernel configuration does not
> have the CONFIG_CLONEFD option enabled.
> EMFILE flags included CLONE_FD, but the new file descriptor would
> exceed the process limit on open file descriptors.
> ENFILE flags included CLONE_FD, but the new file descriptor would
> exceed the system-wide limit on open file descriptors.
> ENODEV flags included CLONE_FD, but clone4() could not mount the
> (internal) anonymous inode device.
> clone4() is Linux-specific and should not be used in programs intended
> to be portable.
> clone(2), epoll(7), poll(2), pthreads(7), read(2), select(2)
> Linux 2015-03-01 CLONE4(2)
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